[Issue 850] Add a control for unfocused window opacity.

Adds debug setting FloaterUnfocusedBackgroundOpaque
Adds "Make unfocused floaters opaque" to Vanity->Main preferences.

One line patch to llfloater.cpp

indra/newview/skins/default/xui/en-us/panel_preferences_ascent_vanity.xml

@@ -11,6 +11,7 @@
       <check_box bottom_delta="-20" control_name="TurnAroundWhenWalkingBackwards" follows="top" height="16" label="Turn around when walking backwards" tool_tip="Certain AOs may turn you around even with this disabled." name="turn_around"/>
       <check_box bottom_delta="-20" control_name="AnnounceSnapshots" follows="top" height="16" label="Announce when someone takes a snapshot" tool_tip="Won't announce for people who hide the fact that they are taking a snapshot." name="announce_snapshots"/>
       <check_box bottom_delta="-20" control_name="AnnounceStreamMetadata" follows="top" height="16" label="Announce music stream's metadata in local chat when tuned in" tool_tip="When a new song comes on, a message will be displayed in local chat with available information about the track." name="announce_stream_metadata"/>
+      <check_box bottom_delta="-20" control_name="FloaterUnfocusedBackgroundOpaque" follows="top" height="16" label="Make unfocused floaters opaque (requires refocusing each floater to take effect)" tool_tip="When a floater loses focus, it won't turn transparent with this enabled.  This may conflict with some skins." name="unfocused_floaters_opaque"/>
     </panel>
     <panel border="true" left="1" bottom="-190" height="180" width="500" label="Tags/Colors" name="TagsColors">
       <!-- Client tag options -->